There isn't necessarily a "correct" way.
- The user needs to understand what he needs to do.
- The CTA must be in a place that is easily seen and follows the user's train of thought as he's completing the form. Therefore the CTA can be anyplace in the form.
- The label (and icon if included) must be informative.
Often times companies ask for attachments at the end of the form but that isn't "necessary", it's convention.
The best answer is to ask your users. A/B test your designs. Show variations to as many appropriate people as possible: receptionists in your office and others, people in the coffee shop. The creation of mock-ups is fast and easy and the time and cost for asking is a minimal.
I for one find the folder less intuitive than an icon of a file. (But that's me.)